<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <button class="btn">点击</button>
  <script>
    // 按需加载的效果 点击按钮后在导入某个文件
    document.querySelector('.btn').onclick = function(){
      // es6提供了import函数 (异步函数)
      import('./index.js').then(res=>{
        console.log(res);
      })
    }
  </script>
</body>
</html>